An unified neutral-point voltage controller for NPC converter fed induction machine drive

The control of the neutral-point (NP) voltage is one of the major challenges in the operation of the neutral-point-clamped (NPC) three-level converter. Some controllers are proposed in the literature which are based on the continuous system model. However, those models are function of either active or reactive power which limits their operation under certain operating point. As an example, the model with the active does not work properly for an induction machine drive under no load condition. In this paper, a hybrid modulation strategy is proposed which causes the NP current to be proportional to the sum of the active and reactive power. Using this, the dynamics of the NP voltage difference is modeled as a function of the sum of the active and reactive power. This approach is used to improve the performance of NPC converter feeding the induction machine under no load condition. In addition, a method to extend the limit of the control variable is also described. Simulation results are shown to verify the proposed modulation strategy and the controller based on that.